The present invention relates to extracorporeal blood processing. More particularly, this invention relates to a method and apparatus for the automated sampling of the hematocrit of blood.
The collection and reinfusion of a patient""s own blood, referred to as autologous blood, offers a number of recognized benefits. For example, the use of autologous blood reduces concerns relating to the possibility of disease transmission via donor blood transfusions, referred to as homologous blood, as well as concerns regarding febrile/allergic reactions. Additionally, autologous blood recovery ensures the rapid availability of blood and reduces or eliminates the need for compatibility testing on such blood since the procedure is most typically completed in an operating room. Further, the use of autologous blood provides red blood cells which are generally superior in quality to banked blood and reduces any quantity of homologous blood otherwise needed. The use of autologous blood is also desirous to many patients for psychological and/or religious reasons.
Intraoperative procedures where autologous blood salvage is currently employed include cardiac and vascular surgery. Specialties which employ autologous blood salvage include orthopedics, plastic and reconstructive surgery, neurosurgery, solid organ transplants, general surgery, gynecology and trauma.
In a typical blood salvage procedure, blood is removed from or about a surgical site via a hand-held suction device, mixed with an anticoagulant, and transferred to a reservoir for subsequent transfer for batch processing. In connection with such collection/transfer of salvaged blood, the blood is typically filtered to remove debris and defoamed to remove entrained gaseous components (e.g. air) utilizing separate in-line filter devices and defoaming devices through which the salvaged blood is sequentially transferred. During batch processing, the salvaged blood and a wash solution are separately pumped in sequence through a centrifuge to separate red blood cells and achieve a degree of washing. Following processing, the red blood cells are removed from the centrifuge for reinfusion to the patient.
Another blood processing procedure is apheresis. Apheresis is a procedure of separating the blood into its various components via centrifugation. By appropriate adjustment of the separation instrument, a selected portion of the blood can be recovered while the rest of the blood is returned to the donor. Examples include plasmapheresis, plateletpheresis and red blood cell collection.
Plasma sequestration is a procedure which may include both plasmapheresis and plateletpheresis. Plasmapheresis is a process whereby plasma containing few cellular components (i.e., platelet poor plasma) is collected. Plateletpheresis is a process whereby plasma containing many cellular components (i.e., platelet rich plasma) is collected. This process also commonly uses centrifugation as its separation mechanism. The plasma component as well as the cellular components may be returned to the patient at the appropriate time.
In blood salvage and red cell collection procedures the objective is to obtain a high quality and amount of red blood cells in the least amount of overdue procedure time. A way to monitor the quality of the procedure is by measuring the hematocrit of the processed blood and maintaining it in a desired range. Methods of doing this include those disclosed in U.S. Pat. No. 5,385,539 (Maynard), which describes a hematocrit sensor used in a plasma separator apparatus. The hematocrit sensor continuously monitors the hematocrit level of processed blood, and adjusts the parameters of the plasma separator apparatus to compensate for a hematocrit that is too high or too low. However, many problems remain in this area, including, for example, variability in the measured hematocrit due to loss of hematocrit sampling data caused by non-blood substances such as air bubbles, or other obstructions in the blood flow path (e.g. clots, fat, etc.) that interfere with the detector signal. In addition, operator error remains a problem with such blood separation and measurement/analysis systems.
This invention provides for accurate measurement of hematocrit during blood recovery or plasma sequestration processes by means of algorithms to account for the presence of air or other non-blood substances in salvaged or separated blood. This invention also provides a method of automatic start-up of the hematocrit measurement circuit. Further, this invention is a method of determining the minimum volume of blood required to obtain an accurate measurement of the hematocrit.
In one aspect, this invention is a method of determining the average hematocrit of blood processed in a blood processing system, comprising providing a blood processing system having a blood inlet and a blood outlet, the blood processing system being sized to process a known volume of blood during a processing cycle; providing a hematocrit measurement circuit connected to measure the hematocrit of blood flowing through the outlet of the blood processing system, the hematocrit measurement circuit having a display for reporting hematocrit values; determining a minimum volume of the known volume of blood which must be measured for hematocrit in order for an average hematocrit value for the known volume of blood to be calculated within a predetermined degree of accuracy; evacuating processed blood from the system through the blood outlet; making a plurality of hematocrit measurements of the processed blood; calculating from the plurality of hematocrit measurements an average hematocrit value for a first volume of processed blood; and comparing the first volume to the minimum volume.
The method may further include reporting the average hematocrit of the known volume of processed blood if the first volume is at least as great as the minimum volume.
In another aspect, this invention is a method of measuring hematocrit in processed blood comprising providing a blood processing system having a blood inlet and a blood outlet, the blood processing system being sized to process a known volume of blood during each processing cycle; providing a hematocrit measurement circuit having a hematocrit sensor connected to measure the hematocrit of blood flowing through the outlet of the blood processing system, the hematocrit measurement circuit having a known lock time after which the circuit first responds to the presence of blood sensed by the sensor; evacuating processed blood from the system through the blood outlet; making a plurality of hematocrit measurements of the processed blood; assigning a hematocrit value to a first volume of processed blood passing the hematocrit sensor during the lock time; and calculating from the plurality of hematocrit measurements and assigned hematocrit value an average hematocrit value of the known volume of processed blood evacuated from the system.
The assigning step may comprise using an algorithm that calculates the hematocrit value based on at least one of the plurality of hematocrit measurements. The step of using the algorithm to calculate the hematocrit value comprises assigning to the first volume of processed blood a hematocrit value equal to the first processed blood hematocrit measurement after the lock time. The step of using the algorithm to calculate the hematocrit value may comprise determining a slope of the first N processed blood hematocrit measurements after the lock time and extrapolating backwards using the slope to assign a hematocrit value to the first volume of processed blood.
The method further includes determining the presence of non-blood substances in the processed blood from the plurality of hematocrit measurements and assigning a hematocrit value to a second volume of processed blood, the second volume being determined from hematocrit measurements of non-blood substances in the processed blood. The step of assigning a hematocrit value to the second volume of processed blood comprises using an algorithm that calculates the hematocrit value based on at least one of the plurality of hematocrit measurements. The step of using an algorithm to calculate a hematocrit value for the second volume of processed blood may comprise assigning to the second volume of processed blood a hematocrit value equal to the average of the last processed blood hematocrit measurement prior to the non-blood substance and the first processed blood hematocrit measurement after the non-blood substance.
In a third aspect, this invention is a method of measuring hematocrit in processed blood comprising providing a blood processing system having a blood inlet and a blood outlet; providing a hematocrit measurement circuit connected to measure the hematocrit of blood flowing through the outlet of the blood processing system; evacuating processed blood from the system through the blood outlet; making a plurality of hematocrit measurements; determining the presence of non-blood substances in the processed blood from the hematocrit measurements; assigning a hematocrit value to a volume of processed blood based on hematocrit measurements of non-blood substances in the processed blood; and calculating from the measured hematocrit and assigned hematocrit values an average hematocrit value of process blood evacuated from the system.
The hematocrit measurement circuit may include a hematocrit sensor connected to measure the hematocrit of blood flowing through the outlet of the blood processing system and wherein the hematocrit measurement system has a known lock time after which the circuit first responds to the presence of blood sensed by the sensor, the method further including assigning a hematocrit value to a second volume of processed blood passing the hematocrit sensor during the lock time.
In a fourth aspect, this invention is a blood processing system comprising a blood processor having an inlet and an outlet, the blood processor being sized to process a known volume of blood during a processing cycle; a hematocrit sensor connected to measure hematocrit of process blood flowing through the outlet of the blood processor at a predetermined sampling rate, the sensor being configured to provide data indicative of the hematocrit of the process blood being sensed and to provide data indicative of non-blood substances when substances other than blood are being sensed; and a controller connected to receive the data from the sensor and to calculate from the data an average hematocrit value of the known volume of processed blood, the calculation being performed in a manner that makes adjustments to the calculation depending on the presence of non-blood substances in the processed blood.
In a fifth aspect, this invention is a blood processing system comprising a blood processor having an inlet and an outlet; a hematocrit sensing circuit including a hematocrit sensor configured to connect with a first connector when the blood processor is not in use and with a second connector when the blood processor is in use, the second connector being connected to the outlet of the blood processor, the hematocrit sensing circuit being switchable from an operational mode when the sensor is making hematocrit measurements and a non-operational mode when the sensor is not making hematocrit measurements, the hematocrit sensing circuit being further configured to be switched to the operational mode when the sensor is disconnected from the first connector.
In a sixth aspect, this invention is a blood processing system comprising a blood processor having an inlet and an outlet; a hematocrit sensing circuit including a hematocrit sensor configured to connect with a first connector when the blood processor is not in use and with a second connector when the blood processor is in use, the second connector begin connected to the outlet of the blood processor, the hematocrit sensing circuit being switchable from an operational mode when the sensor is making hematocrit measurements and a non-operational mode when the sensor is not making hematocrit measurements, the hematocrit sensing circuit being further configured to be switched to the operational mode when the sensor is connected to the second connector.